Several anti-trip device systems can be found in patent literature. One of these was described in the work entitled, “Der Chronometer Gang” by Professor Alois Irk and published by Deutsche Uhrmacher Zeitung, Berlin 1923. Reference can be made in particular to paragraphs 116 to 120 (pages 74 to 77) and to FIG. 25 of the aforecited work. This device is implemented in an escapement comprising amongst other things a balance spring formed of several coils and a balance provided with at least one arm, the balance being pivotably mounted between a plate and a bridge. The device comprises a finger fixed on the balance arm, two columns between which the finger can pass when the balance is in movement, these columns being secured to the balance bridge, and a locking arm fixed to the external coil of the balance spring. The locking arm is able to be inserted between the columns and the finger to prevent the balance from rotating beyond an angle exceeding its normal operating angle.
This device is implemented in so-called detent escapements which are suited to timepieces of large dimensions such as marine chronometers. These timepieces are appreciated for their high level of precision, which is why a detent escapement, which is itself known for its high precision, is often used. This escapement has, however, a significant defect, namely its sensitivity to shocks. Consequently, it is known to be unsuitable for wristwatches. Indeed, a shock applied to the timepiece can cause its balance to rotate beyond a normal operating angle. This then produces a trip, at least for one direction of rotation of the balance, since two unlockings and two pulses occur during the same vibration.
When one wishes to fit a timepiece of small dimensions, for example a wristwatch, with a detent escapement in order to replace, for example, a conventional Swiss lever escapement, and thus enable the timepiece to enjoy the advantages conferred by this escapement, new techniques, different from those known to date will have to be used if one wishes to avoid failure. Various solutions have been proposed recently for compensating for the lack of energy developed by the sprung balance of a wristwatch to overcome the forces acting on the detent of a detent escapement. One solution is explained, for example, in EP Patent No. A 1 538 491. Nonetheless the problem of tripping remains and this has to be solved when a balance spring of small dimensions, such as that mounted in a wristwatch, is involved.
The solution proposed by Alois Irk described hereinbefore could be applied to a wristwatch but seems to have straight away at least two drawbacks. The first drawback appears to be the use of a locking element fixed to a resilient element, in this case the external coil of the balance spring. This is not a very mechanical solution, subject to all kinds of unanticipated unknowns linked to the resilience of the balance spring, which might deform in an unexpected manner, and precisely following a shock applied to the watch. The second drawback lies in the fact that the Irk system only operates in one rotational direction of the balance, the direction of the largest expansion of the balance spring. In the other direction, the direction of contraction of the balance spring, the escapement can also trip and the Irk system thus remains inefficient.